Business and consumers use a wide array of wireless devices, including cell phones, wireless local area network (LAN) cards, global positioning system (GPS) devices, electronic organizers equipped with wireless modems, and the like. The increased demand for wireless communication devices has created a corresponding demand for technical improvements to such devices. Generally speaking, more and more of the components of conventional radio receivers and transmitters are being fabricated in a single integrated circuit (IC) package. In particular, single chip radios with no off-chip channel filters are very popular. Direct conversion receivers and software-defined radios are being developed in order to simplify single chip designs and to make each design suitable for as many applications as possible.
There are two main classes of radio architectures that can eliminate channel filters: 1) zero-IF architectures and 2) low-IF architectures. In these approaches, the channel filtering and amplification functions are performed in the low frequency band using on-chip mixed-signal processing techniques. However, due to physical impairments in some of the receiver building blocks, such as DC-offset and low frequency noise, zero-IF and low-IF architectures tend to have inferior performance when compared to conventional SAW filter-based super-heterodyne receivers.
FIG. 1 illustrates conventional zero-IF radio receiver 100 according to one embodiment of the prior art. Zero-IF radio receiver 100 comprises low-noise amplifier (LNA) 105, sine and cosine generator 110, in-phase (I) channel mixer 120A, quadrature (Q) channel mixer 120B, in-phase (I) channel filter and automatic gain control (AGC) block 130A, and quadrature (Q) channel filter and automatic gain control (AGC) block 130B. Sine and cosine generator 110 receives a local oscillator (LO) reference frequency signal and generates an in-channel (I) LO reference signal and a 90 degree phase-shifted quadrature (Q) LO reference signal. The in-channel LO reference signal is applied to one of the inputs of in-phase channel mixer 120A and the 90 degree phase-shifted quadrature LO reference signal is applied to one of the inputs of quadrature channel mixer 120B.
Low noise amplifier (LNA) 105 amplifies the incoming RF signal from the antenna and applies the amplified RF output signal to one of the inputs of in-phase (I) channel mixer 120A and one of the inputs of quadrature (Q) channel mixer 120B. In-phase channel mixer 120A mixes the in-channel LO reference signal and the amplified RF output signal to produce a down-converted I-channel baseband signal. Quadrature channel mixer 120B mixes the 90 degree phase-shifted quadrature LO reference signal and the amplified RF output signal to produce a down-converted Q-channel baseband signal. The down-converted I-channel baseband signal is filtered and amplified to a suitable level by I-channel filter and AGC block 130A to produce an I-channel baseband output signal. The down-converted Q-channel baseband signal is filtered and amplified to a suitable level by Q-channel filter and AGC block 130B to produce a Q-channel baseband output signal.
Zero-IF radio receiver 100 uses baseband frequencies for the receiver channel filtering and amplification functions. The conversion from antenna signal to baseband signal can be achieved in one or more down conversion mixing steps. If, as shown in FIG. 1, the mixing is a one-step process (i.e., from antenna signal directly to baseband signal), then the radio is a true direct-conversion radio. It is noted that a direct-conversion receiver is also a zero-IF receiver. However, a zero-IF receiver may not necessarily be a direct-conversion receiver. The baseband signal at the output of the down conversion mixer is equivalent to the transmitted baseband signal, except that the signal level is very small and mixed with many unwanted interfering signals at extremely high levels (e.g., ≧60 dB higher than the useful signal).
These unwanted interfering signals must first be attenuated by a baseband filter in I/Q-channel filter and AGC block 130 before amplifying the useful signal. Otherwise, signal clipping occurs. The zero-IF architecture shows advantages in this process because precision amplifiers and high-Q filters can be easily built on a single silicon substrate at these baseband frequencies.
However, physical impairments, such as DC offset and low frequency noise, limit the feasibility of implementing I/Q-channel filter and AGC block 130 for some applications. For example, the GSM cellular phone outputs less than 30 millivolts (peak-to-peak) at the mixer output. The equivalent noise levels caused by the DC offset, low-frequency noise, and other impairments are in the range of hundreds of millivolts (mV). Advanced adaptive control loops may be used to cancel out these impairments. However, due to a large difference in signal magnitudes, the final circuit is either too costly to implement or has poor dynamic performance.
In addition, the spectrum of the baseband signal is symmetrical about the DC frequency (i.e., 0 Hz), which causes overlapping sidebands. To extract the complex digital data out of the sideband-overlapped signal, the down conversion requires a quadrature mixer, which is a combination of two mixers (i.e., mixers 120A and 120B) sampled with sine and cosine local oscillator (LO) signals, as shown in FIG. 1. The quadrature mixing is still a one-step process, so it is not disqualified from being a direct-conversion type receiver. However, following the quadrature mixer, two channels of filter/amplifier are required. This is one major disadvantage of implementing a zero-IF receiver.
I-channel filter and AGC block 130A and Q-channel filter and AGC block 130B are identical. Each channel has a low-pass filter followed by a variable gain (AGC) amplifier. The amount of filtering and amplification depends on the type of radio for which the receiver is designed and the amount of digital signal processing (DSP) power placed after I-channel filter and AGC block 130A and Q-channel filter and AGC block 130B.
FIG. 2 illustrates conventional low-IF receiver 200 according to one embodiment of the prior art. Low-IF radio receiver 200 comprises low-noise amplifier (LNA) 210, image rejection mixer 220, bandpass channel filter and automatic gain control (AGC) amplifiers block 230, and signal digitizer and digital signal processing (DSP) block 240. Signal digitizer and DSP block 240 comprises sine and cosine generator 260, in-phase (I) channel mixer 250A, and quadrature (Q) channel mixer 250B.
Unlike zero-IF receiver 100, low-IF receiver 200 requires at least two mixing steps to yield the baseband signal. The amplified signal from LNA 210 first goes through a down-conversion in image rejection mixer 220 that shifts the original radio frequency (RF) carrier to a low-frequency carrier. After the down-conversion, the signal remains in its original modulated form, which exhibits a bandpass characteristic. The signal level is very small and mixed with many unwanted interfering signals at extremely high levels (e.g., ≧60 dB higher than the useful signal.)
These unwanted interfering signals must first be attenuated by a bandpass filter in bandpass channel filter and AGC amplifiers block 230 before amplifying the useful signal. Otherwise, signal clipping occurs. Low-IF receiver 200 shows disadvantages in this process because the unwanted interference signals appear at both sides of the frequency band of the useful signal. To reject these interfering signals, a bandpass filter must be used instead of a lowpass filter. The design of the bandpass filter must also consider the frequency fold-back effect, which may substantially degrade the filter performance. This problem arises because the frequency of the carrier is not high enough and there is insufficient frequency space to hold the spectrums of its sidebands and other unwanted interfering signals. The frequency space is the difference between the frequency of the carrier and 0 Hertz. The fold-back phenomenon depicts the overlay of frequencies of the unwanted interfering signals onto the useful signal spectrum. This disadvantage is not applicable to zero-IF receivers.
However, low-IF receiver 200 does not generate overlapping sidebands because of the use of a carrier frequency. Therefore, the down-conversion mixer and bandpass channel filter and AGC amplifiers block 230 can be a simple one channel implementation (i.e., not quadrature). After bandpass channel filter and AGC amplifiers block 230, the low-IF signal is digitized before feeding the downstream DSP or hardwired logic blocks. These downstream digital blocks execute the second step of demodulation: utilizing a quadrature mixer to convert the low-IF signal to the baseband signal. In general, these digital blocks are trouble-free implementations such that the selection of receiver architectures is seldom affected by these blocks.
Although the single channel filter and amplifier requirement offers cost advantages, the design of a low cost low-IF receiver 200 is still a challenging task because the highpass corner frequencies of the bandpass filter are relatively low, thus requiring large capacitance values. On the lowpass side, the filter requires additional poles to achieve the required attenuation because the operating frequency is several times higher than that of an equivalent filter in zero-IF receiver 100.
Unfortunately, low-IF receiver 200 is not entirely insensitive to DC offset and low frequency noise. Because the lower sideband of the shifted carrier signal is close to the DC frequency (0 Hz), the bandpass filter has to provide some gain at these low frequencies for proper signal demodulation. This non-zero gain property causes low-IF receiver 200 to respond to DC-offset fluctuations and low frequency noises. This can be a major concern when implementing high sensitivity receivers.
Also, a DC-offset cancellation may still be conditionally required when implementing low-IF receiver 200. The reason is that there is a need for cascading many amplifiers to achieve the required system gain. In spite of the size and cost of the low frequency highpass filters, some of these amplifiers may have to use direct coupling instead of using all highpass coupling throughout the amplifier chain. To achieve good blocking characteristics, DC-offset cancellation blocks are thus included to avoid signal clipping.
Lastly, there is a unique problem associated with low-IF receiver 200: the image rejection problem. Since low-IF receiver 200 uses a low IF frequency on the order of channel bandwidth, image frequencies occur very close to the received signal frequency. These image frequencies are either within, or close to the received band, so that low-IF receiver 200 provides very poor or no rejection at these frequencies. To combat this, image rejection mixer 220 must be used.
Therefore, there is a need in the art for an improved RF receiver architecture that has all of the advantages of the existing approaches but none of the disadvantages. In particular, there is a need for an RF receiver architecture that can achieve a high degree of integration that does not suffer from the problems inherent in conventional zero-IF and low-IF receivers.